1 Answer
1

root is the device you want to be mounted up as the root file system when the kernel first starts. This is pretty self explanitory, but it gets complicated because this can actually change over time. The usual reason that happens is when the kernel doesn't have the modules it needs to mount the root file system. In that case a system called initrd is used. An initrd image is basically a small compressed file system with a few goodies like drive controller or network modules that the kernel is going to need to read the real root file system and continue booting. In this case the initrd image becomes root, and...

real_root is going to be the actual root partition matching your entry in /etc/fstab. If you don't use initrd, this option can be omitted in favor of just using root. As long as we're on the topic, there is also nfsroot which is specifically for situations where the root file system will be an NFS mounted remote file system and networking needs to be initiated before the final root file system can be mounted.

cdroot I don't recognize, but it probably has to do with the special way your Live distro is setup and would denote where to find the LiveCD/Image as opposed to the virtual file system or that is the root of the running live distro. In searching it seems to show up mostly with Gentoo LiveUSB/CD builds, so it may be proprietary. It often does not have an argument, so it might simply be a flag to denote that the root media is a CD so that later processes can know.

@Franciois: Be sure to check back, you might get some even better answers. There are people here that know more than I do about that area in general and Gentoo in specific, so don't run too far away :)
–
CalebJul 5 '11 at 15:26